1. Field of the Technology
The present disclosure is directed to a method involving the use of cation exchange material for producing ammonium metatungstate. According to certain embodiments of the method, a high purity crystalline ammonium metatungstate may be formed from ammonium paratungstate.
2. Description of the Background of the Technology
Ammonium paratungstate (APT) and ammonium metatungstate (AMT) are used as tungsten-containing raw materials in the catalyst industry. APT has limited solubility in water and typically has been converted to a more soluble peroxytungstate form for use in catalyst production. AMT is readily soluble in water, but is more expensive than APT because it is produced through difficult synthetic routes and with low yields. In the production of certain catalysts, aqueous solutions of the tungstate compounds are prepared and adsorbed onto alumina or another solid support. The adsorbed tungstate is then converted to a catalytically active tungsten, tungsten oxide, or tungsten carbide.
Several processes for producing AMT are disclosed in the prior art. U.S. Pat. No. 3,175,881 discloses one process for producing AMT from APT. APT is first calcined to drive off ammonia and water vapor. The calcined material is digested in 80° C. water, while preferably maintaining pH in the range of about 3.5 to 4.0. Additional ammonia is driven off during the digesting process, and water is evaporated to concentrate the solution. Insoluble tungsten oxide may be produced, and some APT remains unconverted. This process produces AMT in a yield of about 75%.
U.S. Pat. No. 3,591,331 discloses a method for producing AMT from an aqueous ammonium tungstate (AT) solution using a solvent. The method includes contacting an AT solution with an organic extractant solution. The extractant solution includes di-2-ethylhexylphosphoric acid and water insoluble hydrocarbon solvent, which extracts ammonium ions from the aqueous solution and lowers the pH to at least about 4.5. The aqueous solution is removed from the organic solution and heated for a least one hour. An essentially pure AMT reportedly is recovered, and the method also reportedly avoids the formation of APT. U.S. Pat. No. 3,857,928, however, reports that the organic phase used in the method tends to be unstable, and that up to 10% or more of the total solid product may be insoluble phosphor-tungstates.
U.S. Pat. No. 3,857,928 discloses a process for producing crystalline AMT from AT solution using an ion exchange column that contains a weak acid cation exchange resin. The AT feed solution is passed through the ion exchange resin until a pH of 3.5 is attained, at which point the effluent is collected. The collected effluent, which includes some metatungstate species, must be digested at a temperature of at least about 98° C. for at least about 5 hours to obtain substantially complete conversion of AT to AMT.
U.S. Pat. No. 3,857,929 discloses a batch process for producing AMT from AT. The process involves introducing a strong acid cation exchange resin containing sulphonic acid groups into an AT solution until a pH of about 3.5 is reached. The resin is removed by filtration, and the filtered solution is digested at about 98° C. for at least about 5 hours to obtain substantially complete conversion of AT to AMT. Crystallization of AMT may be carried out in a conventional manner.
A method for producing AT, APT, AMT, or hydrated tungsten trioxide is disclosed in U.S. Pat. No. 3,936,362. The process includes passing tungstate anions through an anion exchange membrane into an aqueous solution containing ammonium cations under the driving force of an electrical potential for a time sufficient to achieve a pH within the range at which the desired tungsten compound will form.
U.S. Pat. No. 3,956,474 discloses an AMT production technique in which an AT solution is digested in the presence of silica. Digestion takes place for at least 4 hours at a temperature of at least 98° C., followed by filtration to remove the silica from the AMT solution. Typically, the AMT solution includes about 0.4% by weight of silica after filtering.
U.S. Pat. No. 4,283,257 discloses a process for producing AMT from APT using an inert liquid permeable medium and direct electric current. APT is introduced in the anode chamber of an electrolytic cell, which is separated from the cathode chamber by an inert liquid permeable media. When direct electrical current is passed through the cell, ammonium ions migrate to the cathode compartment, resulting in an increased concentration of metatungstate in the anode chamber.
Digestion of calcined APT to form an AMT solution is disclosed in U.S. Pat. No. 4,504,461. The calcining process includes fluid bed roasting APT at 275° C. to 305° C. to form a precursor of AMT. The AMT precursor is slurried and digested for 30 to 120 minutes at 70° C. to its boiling point. The pH of the resulting slurry is between 3.0 and 4.0, and may be controlled in that range for consistent results.
U.S. Pat. No. 4,557,923 also discloses a method for producing AMT by roasting APT. APT is roasted at 275° C. to 300° C. for 10 to 20 hours. The roasted APT is added to a dilute solution of AMT having a pH of 3.6 to 4.6. The roasted APT is added at a rate suitable to maintain the pH of the AMT solution at 3.6 to 4.2. After addition of all of the roasted APT, the pH of the slurry is stabilized at 3.6 to 4.2. After dissolution, the resulting AMT solution is concentrated and insoluble materials are removed. Based on the WO3 content of the starting APT, the reported yield of AMT may be at least about 97%.
U.S. Pat. No. 4,612,180 discloses a method involving first heating APT using microwave radiation. A relatively constant volume of an aqueous slurry of the microwave-heated APT is heated at about 80° C. to about 100° C. until slurry pH stabilizes at 5.8 to 6.8. The slurry pH is adjusted to 4.2 to 3.0 by addition of either dilute mineral acid or a strong acid cation exchange resin. The cation exchange resin is then removed from the pH-adjusted slurry, and the slurry is digested at 80° C. to 100° C. for 2 to 6 hours to form an AMT solution. The AMT solution is concentrated by evaporation, and crystalline AMT is obtained in a conventional manner. Based on the WO3 content of the starting APT, the yield of AMT reportedly is at least about 93%.
U.S. Pat. No. 4,612,182 discloses an AMT production technique including heating APT at 100° C. to 250° C. for 1 to 8 hours to drive off at least some portion of ammonia and water from the APT. The heated APT is digested in water at 80° C. to 100° C. for 2 hours to 6 hours at relatively constant volume, while maintaining the pH at 4.2 to 3.0 by addition of ammonia, if necessary. A solution including AMT and water is formed. The AMT solution is concentrated and filtered, and AMT is crystallized. Based on the WO3 content of the starting APT, the AMT yield of the technique is reportedly about 95%.
Despite the several existing techniques for producing AMT, a need has existed for a less expensive, high yield process for producing AMT.